Feature request: Ability to add custom type visitors

It would be very nice to be able to add a custom visitor, for when you have a custom type, that needs special processing, or for fixing/working around issues before a new release with a new type visitor is released.

And maybe even, being able to override an existing type visitor.

The issue names no files, tests, or entry points. Start by locating the type visitor implementation and how visitors are registered, then clarify the extension and override behavior with maintainers. Done means custom visitors can handle custom types and the intended precedence for overriding existing visitors is covered by tests.

Written by the indexing model from the issue text.